Gentamicin serum levels were determined in an unselected group of newborns treated with standard doses of the drug to study the changes in pharmacokinetic parameters (trough and peak serum levels/dose ratio, serum level increases/dose ratio, elimination half-life, and distribution volume) 48, 96, and 144 h after commencing treatment. However, no significant changes in the pharmacokinetic parameters were found in different groups of neonates, classified according to gestational age and days of life during the study period. It appears that the dose of gentamicin once it has been individually established does not have to be changed, at least during the first week of treatment.